The goal of any trunked communication system is to allocate a limited number of communication channels among a plurality of subscriber units. To establish communication between subscribers, it is known for a requesting subscriber unit to transmit a call request code over a particular one of the communication channels reserved as a control channel. Upon receipt of the call request, the central returns a channel grant code (assuming that a channel is available for use), which instructs the requesting subscriber to move to a particular communication channel as a transmitting unit and instructs other subscribers to move to the communication channel as receiving units.
Generally, trunked communication channels comprise a pair of frequencies: an outbound frequency and an inbound frequency. The inbound frequency is typically understood to carry communications from the subscriber to the central station, while the outbound frequency carries communication from the central to the subscriber units. As used herein, subscriber units may be portable units, mobile units, or control stations. Generally, a portable unit is a transceiver design to be carried on or about the person, a mobile unit is a transceiver design to be installed in vehicles, and a control station is generally understood to be a permanent or semi-permanent installation in a building or other fixed site.
In trunked communication systems, access time is defined as the time interval between the time the requesting subscriber generates its call request, until the time the receiving subscriber units may receive a valid transmission on the assigned communication channel. Clearly, it is advantageous to make the access time of the trunked communication system as short as possible. One existing trunked communication system has an access time of approximately 0.5 seconds. While this has proved sufficient for the communication needs of several trunked system users, the public safety market has collectively published the so-called APCO-16B specification, which requires a trunked communication access time of less than 0.5 seconds. Therefore, while the existing access times may generally meet the APCO-16B specification, there is a need in the art to dramatically improve the access time of trunked communications systems for the public safety market.